Reproductive traits are among the most important biological characteristics that facilitate or impede the establishment of non-native fish into new environments. In several tributaries of the Lower River Danube (Bulgaria), the round goby (Neogobius melanostomus Pallas, 1814) is a recent invader. Specimens from the edge of the invasion front were collected monthly (March 2017 to May 2018) from each river. The pioneer individuals displayed relatively small body sizes. First maturation of females occurs at 49 mm total length. Based on the dynamics of oocyte size and GSI, it was established that spawning begins in early spring, at a relatively low water temperature (6-7 °C). The breeding season extended from March to June. Average absolute fecundity was 162 ± 62 oocytes, while average relative fecundity was 94 oocytes/g ± 24. Female fecundity is linearly dependent on the length and weight. Both of the known male alternative reproductive tactics were observed. Body condition factor was lowest during April for both sexes. The relationship between reproductive traits and invasive potential in N. melanostomus is discussed.

The impacts of aquatic invasive species vary from the population to ecosystem level most strikingly through modifications to native communities, often leading to a decline in native species. A primary impact mechanism is competitive displacement of native by invasive species through resource partitioning. However, the trophic interactions between native and invasive species occupying the same habitat remain poorly understood, particularly at the early stages of invasion. This study used stable isotope analysis of two co-occurring populations of invasive topmouth gudgeon, Pseudorasbora parva and native Caucasian dwarf goby, Knipowitschia caucasica in a highly productive shallow lake to characterize overlap of potential trophic niches. The trophic niches of both species were divergent, with no overlap. Mixing models suggest some inter-specific dietary differences. The trophic niche of the Caucasian dwarf goby was slightly and non-significantly larger than that of topmouth gudgeon. These results suggest that when introduced outside of their natural range, topmouth gudgeon might integrate into new fish communities via the exploitation of resources that are underexploited by native fishes, which could also explain the high invasion success of the species.

Alongside climate change, the introduction of non-native species (NNS) is widely recognized as one of the main threats to aquatic biodiversity and human wellbeing. Non-native species and biodiversity are generally low priority issues on the political agendas of many countries, particularly in European countries outside the European Union (EU). The objectives and tasks of this study were to address the policy regulation, education level, education practices, and socioeconomic perceptions of NNS in the Balkans. A questionnaire-based survey was conducted in Albania, Bosnia and Herzegovina, Montenegro, North Macedonia and Turkey (Balkan EU candidate and potential candidate members), in Croatia and Greece (Balkan EU Member States) and Italy (non-Balkan EU Member State). The EU Alien Regulation (1143/2014) concerning NNS is implemented in EU Member States and Montenegro, whereas Albania, Bosnia and Herzegovina and Turkey have not reported specific policy regulations for NNS. Permanent monitoring programmes specifically designed for NNS have not yet been established in the EU Member States. Most countries tackle the issue of NNS through educational activities as part of specific projects. Education level is indicative of the implementation of NNS policy regulation, and efforts are needed for the proper development of relative study programmes. Public awareness and educational preparedness concerning NNS in the Balkans were identified as poor. Strong programmes for management and education should be developed to increase public awareness to prevent further biodiversity losses in the Balkan region.

In a recent study, we showed how local-scale climate change impacts (increased temperature, reduced rainfall, shifts in peak rainfall) affected the hydrology of a channelised lowland European river (reduced flow, reduction in flood events, increased siltation, macrophyte growth), allowing native fish species to recolonise the bankside zone and reduce the density of invasive round goby Neogobius melanostomus by effectively removing its preferred habitat, rip-rap bank stabilisation. Here, we report on a follow-on study whereby stretches of the newly vegetated bank were stripped back to clean rip-rap to assess whether presence/absence of rip-rap was the major factor affecting non-gobiid, tubenose goby Proterorhinus semilunaris and round goby abundance. Our results confirmed rip-rap as a major factor increasing round goby abundance, and hence invasion success, on European rivers, while vegetated banks saw an increase in the abundance and diversity of non-gobiid species. While tubenose gobies showed no preference for habitat type, their numbers were significantly reduced in rip-rap colonised by larger and more aggressive round gobies. We discuss our results in light of recent artificial bank restoration measures undertaken on the Danube and Rhine and the potential role of round goby as a flagship species for cost-effective, large scale river bank restoration projects with multiple ecosystem benefits.

Identification of factors that facilitate successful completion of invasion process stages by nonnative species is a major priority among invasion biologists. Stage-based analyses of non-native fish species traits have been conducted for several regions, but not for a subtropical non-native species hotspot like peninsular Florida. Typically, establishment is the first stage of analysis but Florida is home to many nonnative fish species that have successfully reproduced, yet failed to establish. Therefore, we used life history traits and three model types (categorical and regression trees, logistic regression, and discriminant analysis) to predict successful reproduction and establishment by non-native fishes in peninsular Florida. Statistical models for predicting both successful reproduction and establishment suggested parental care was the most important variable, but other traits included in the best models differ between the two stages. The high level of parental care in successful non-native fishes of Florida is unique among non-native freshwater fish faunas across the United States. Other studies also found that suites of traits used to predict various stages of the invasion process differ, suggesting that stage-based analyses provide a good foundation for better understanding invasion processes. Our results may be applied to stage-based risk screening tools for nonnative fishes in Florida.

The tubenose goby has been reported to be the first non-native postglacial gobiid immigrant from the Lower Danube refuges. It is thus a pioneer species that was the forerunner of the extensive invasion of Ponto-Caspian gobies that ascended the River Danube and spread across Europe a century before other goby species. It appears that recently the tubenose goby invasion has accelerated. In this paper historical data on the distribution of the tubenose goby, together with data from extensive monitoring of fish communities in Slovakia are examined to evaluate both the temporal and spatial aspects of tubenose goby distribution dynamics. Until the 1990s, the species was recorded only in the River Danube and small water bodies in the Danubian Lowland (Slovakia). Since then the tubenose goby has spread upstream into tributaries of the River Danube. It was also recorded in several streams in eastern Slovakia after 2014, and the spatial data demonstrate that the tubenose goby has been colonising new water bodies, progressing to the north and ascending rivers, reaching higher altitudes than previously reported. These findings suggest that the tubenose goby deserves attention, even after two centuries following the onset of its invasion, and its further expansion across Europe should be carefully monitored.

Following their first appearance, the invasive fishes Pseudorasbora parva and Perccottus glenii have been in Lithuania for several decades. However, until recently, information relating to their distribution and secondary spread was limited. For this reason, suitable habitats for these fish species were surveyed for their presence across the entire country. Additionally, all previously reported records on the presence of these species were summarized. Results revealed P. glenii to be widely distributed within the country with abundant populations in habitats suitable for the species. The recent distribution of P. parva is restricted to only a few water bodies. It was shown that both species are associated with human mediated transfer, while no natural dispersal of these invasive species was observed. The results of this study suggest that the invasion of Lithuanian inland waters by P. parva and P. glenii is still ongoing, and their occurrence in numerous water bodies, which are still devoid of these species, now seems probable. Demonstrated vectors of P. parva and P. glenii introductions in Lithuania highlight the importance of controlling and screening human activities related to aquaculture, recreational angling and the ornamental fish trade in order to restrict further P. glenii and P. parva expansion in this region.

The invasion success of gibel carp (Carassius gibelio) depends on demographic and competitive traits. The major biological trait responsible for the invasiveness of C. gibelio is the mode of reproduction. Apart from sexual reproduction, which is typical in fish, C. gibelio is a unique cyprinid species able to reproduce through asexual gynogenesis, which is also known as sperm-dependent parthenogenesis, observed in all-female populations. Though the sexual and asexual forms of C. gibelio co-exist widely in natural habitats, the gynogenetic form has the capacity to modulate the range of effective ecological niches, which may facilitate the process of invasion. In this paper, we reviewed current knowledge of the sexual and gynogenetic forms of gibel carp along with their physiological advantages, immunological traits, and ability to withstand different environmental conditions. As parasitic infection may directly alter the immunology of hosts, and also indirectly alter their investment in reproduction, we provide some insights into the role of parasites as one of the potential drivers facilitating the coexistence of asexual and sexual forms. We highlight evidence that gibel carp have been identified as a serious threat to native species; hence, its impact on the ecosystem is also discussed.

Non-native species are known to escape their parasites following introduction into a new range, but they also often acquire local parasites as a function of time since establishment. We compared the parasite faunas of five non-native Ponto-Caspian gobies (Gobiidae) and local fish species (Perca fluviatilis, Gymnocephalus cernua, Gobio gobio) in three European river systems; the Rivers Rhine, Vistula and Morava, where Ponto-Caspian gobies were introduced 4-13 years prior to the study. Overall parasite species richness was considerably lower in non-native gobies compared to local fish species, and the same result was found at the component and infra-community levels. Both parasite abundance and diversity greatly varied among the regions, with the highest values found in the River Vistula (Wloclawski Reservoir), compared to a relatively impoverished parasite fauna in the River Morava (Danube basin). While only half of parasite species found in local hosts were acquired by non-native gobies, most of the parasites found in gobies were shared with local fish species related either phylogenetically (percids) or ecologically (benthic gudgeon), including the co-introduced monogenean Gyrodactylus proterorhini. As a result, similarity in parasite communities strongly reflected regional affiliation, while phylogenetic distances between fish host species did not play a significant role in parasite community composition. In accordance with other studies, all parasites acquired by gobies in their new range were generalists, all of them infecting fish at the larval/subadult stage, indicating the possible importance of gobies in the life cycle of euryxenous parasites. The absence of adult generalists, particularly ectoparasites with low host specificity, in non-native fish may reflect their generally low abundance in the environment, while an absence of adult endoparasitic generalists was probably related to other factors.

An information-theoretic approach was used to evaluate non-native freshwater fish species introduced to insular habitats of Hawaii and Guam comparing successful establishments vs. failures. Since the late 1800s, as many as 81 non-native freshwater fish species have been recorded as introduced to Hawaii and Guam (combined) and 50 (62%) of these are documented as having one or more established populations. We examined eleven independent variables to investigate establishment success by creating 21 a priori logistic regression models ranked using Akaike's Information Criterion adjusted for small sample size. An additional eight post-hoc models were included that comprised the best a priori model and various combinations of individual variables. The best overall model of establishment probability included effects of taxonomic affinity (family membership), prior establishment success on other tropical islands, and hypoxia tolerance. Establishment success in Hawaii and Guam was highest for those species established on many other islands, and according to our best model air-breathing fishes were more likely to become established. Six fish families, each with from three to 18 species introduced to Pacific islands, were highly successful at establishment: Cichlidae (16 established of 18 species introduced), Poeciliidae (seven of eight), Cyprinidae (four of seven), Centrarchidae (four of four), Clariidae (three of three), and Loricariidae (three of four). Those that successfully established include both small and moderately large-bodied taxa, while representing a diverse array of other morphological and life-history traits. Pathways and motives associated with fish introductions in the Pacific have been linked to desires to develop aquaculture, enhance wild stocks of food, sport, and bait fishes, for use as biological control agents, or are linked to the ornamental fish trade. We found that many established species were introduced via multiple pathways (up to eight) and our analyses suggest that the combination of prior establishment success on other tropical islands and presence of non-native fishes in multiple pathways was indicative of high propagule pressure. Our study results and conclusions on Pacific tropical island introductions are in general agreement with previous studies on non-native freshwater fishes in other regions of the world and similar to observations in continental ecosystems and temperate zones.

Invasive carps are ecologically and economically problematic fish species in many large river basins in the United States and pose a threat to aquatic ecosystems throughout much of North America. Four species of invasive carps: black carp (Mylopharyngodon piceus), grass carp (Ctenopharyngodon idella), silver carp (Hypophthalmichthys molitrix) and bighead carp (Hypophthalmichthys nobilis), are particularly concerning for native ecosystems because they occupy and disrupt a variety of food and habitat niches. In response, natural resource agencies are developing integrated pest management (IPM) plans to mitigate invasive carps. Control tools are one key component within a successful IPM program and have been a focal point for development by governmental agencies and academic researchers. For example, behavioural deterrents and barriers that block migratory pathways could limit carps range expansion into new areas, while efficient removal methods could suppress established carp populations. However, control tools are sometimes limited in practice due to uncertainty with deployment, efficacy and availability. This review provides an overview of several emerging modelling approaches and control technologies that could inform and support future invasive carp IPM programs.